The conventional amplifier circuit as shown in FIG. 1 is useful from audio frequencies to several MegaHertz. It comprises an amplifier (A) 100 and a low pass filter (LPF) 103 which consists of a resistance R4 and a condenser Cl, and inputs V.sub.i are fed into a positive side of amplifier A and into the input side of low pass filter LPF, while the output side of the low pass filter LPF is connected to a negative side of amplifier A.
The conventional circuit constituted as above is operated in the following manner.
The characteristics of the output V.sub.o as against the input V.sub.i, i.e., the transfer characteristics can be calculated based on the following formula: EQU V.sub.o =(.sub.V.sub.i -L.sub.p V.sub.i).multidot.G.sub.o ( 1)
where L.sub.p represents a transfer characteristic function of LPF, and G.sub.o represents the gain of output amplifier 100.
Formula 1 can be rewritten: EQU V.sub.o /V.sub.i =(1-L.sub.p).multidot.G.sub.o ( 2)
The transfer characteristics function L.sub.p for the low pass filter LPF can be calculated based on the following formula: ##EQU1##
Formula 3 can be substituted into Formula 2 to obtain the following formula: ##EQU2## In the conventional circuit, the transfer characteristics at f=f.sub.c is 1/.sqroot.2 as can be seen by Formula 4 and FIG. 2.
Here, it is noted that the emphasis circuit shown in FIG. 1 has the transfer characteristics of Formula 4, and the gain characteristics for different frequencies can be expressed as shown in FIG. 2.
Therefore, as shown in FIG. 2, the emphasis circuit operates in such a manner that, the higher the frequency (f) of the signals, the greater the gain.
When a recording is carried out on a VCR tape by FM-modulating the luminance signals using the emphasis circuit of FIG. 1, the noise components are recorded in a higher level proportional to the frequency. To improve the S/N(signal/noise) characteristics during a playing, the luminance signals are emphasized during the recording.
The level of the emphasis as against the frequency is determined by a standard specification, but the specification can be changed in order to vary the quantity of the emphasis for the purpose of improving the picture quality. The conventional circuit cannot be easily adjusted for a new specification, and generally must be redesigned and rewired.